An overhead-valve (OHV) engine includes valves in the cylinder head and cams for driving the valves positioned in a lower portion of the engine. This arrangement results in a small, compact, lightweight design. Because OHV engines are mechanically simple, they can be manufactured inexpensively and maintained easily.
For these reasons, the OHV engine is used widely as a general-purpose engine in applications such as lawn mowers, working vehicles, portable generators, etc. The OHV engine is still used in these fields despite the appearance of overhead-cam (OHC) design engines capable of high revolution speeds and high-outputs.
In general, the larger the aperture areas in the inlet and outlet ports to the combustion chamber, the less restricted will be the air flow into and out of the chamber, resulting in more power and greater efficiency. It is therefore desirable to achieve larger aperture areas for the inlet and exhaust ports. In order to enlarge these ports, it is widely known to provide a hemi-spherical combustion chamber in the cylinder head, and to position an inlet valve and an exhaust valve in such a manner that they incline along the hemi-spherical surface of the combustion chamber. That is, each valve will move perpendicular to a tangent to the curved surface where the respective valve is located.
When the combustion chamber in the cylinder head is thus hemi-spherical, the ignition flame propagation distances can be relatively equal, and the chamber can be kept small. This improves combustion efficiency, and is advantageous for improvement of exhaust gas conditions, also.
In an OHC engine, the cams for driving the inlet and exhaust valves of the engine are positioned in the cylinder head. It is therefore relatively easy to enlarge the aperture areas of the inlet and exhaust ports by forming a hemi-spherical combustion chamber in the cylinder head and positioning the valves there in such a manner that they incline along the hemi-spherical surface of the combustion chamber.
In contrast, the cam shaft in an OHV engine is positioned in a lower portion of the engine. The cam rocks a rocker arm in an upper portion of the engine through a tappet and a push rod, so as to move up and down the inlet valve or the exhaust valve positioned in the cylinder head. In general, the inlet valve and the exhaust valve are positioned in such a manner that the center of the inlet valve and the center of the exhaust valve define a line that is parallel with the cam shaft. Therefore, if the combustion chamber in the cylinder head is hemi-spherical, and the directions in which the valves move are each inclined toward the center of the hemi-spherical chamber in order to enlarge the aperture areas of the inlet and exhaust ports, as is the case with an OHC engine, the direction in which each valve moves will not coincide with the direction in which the associated rocker arm rocks.
As a result, with respect to the direction in which the valves move, torsion is produced in the direction in which the associated rocker arm rocks. Consequently, the operation of a valve mechanism including the inlet valve, the exhaust valve, the rocker arm and the push rod is not smooth, and harmful or useless force is applied to parts of the valve mechanism. Therefore, deflective wear occurs on a support portion of the rocker arm. This can cause deformation in the rocker arm and the push rod in some conditions.
If the valve mechanism were to be made so rigid, strong and/or resistant to wear as to effectively resist deflective wear and deformation, this would increase the weight, size, and complexity of the valve mechanism.
In particular, in the case of an internal combustion engine which rotates at speeds in the thousands of revolutions per minute, the stresses repeatedly acting on the valve mechanism make may it necessary to replace parts of the valve mechanism often.
Japanese Patent Laid-Open Publication H.5-133205 discloses prior art relating to an OHV engine, in which the combustion chamber is hemi-spherical with the inlet and exhaust valves inclining toward the center of the chamber. This mechanism, however, does not solve the technical problems stated above.